Due to the length scales in microfluidic systems interfacial forces dominate over inertia and gravity. In this work, I have designed, fabricated and studied several systems that manipulate interfacial forces for manufacturing and enhance convective heat transfer. These systems have application in drug delivery, biological and chemical micro-reactors, and electronics cooling. First, fluid-fluid interfaces can be used for the generation of particles. At the interface between two fluids in motion, instabilities can occur. One reason for these instabilities is the difference in shear velocities which causes waves to propagate at the interface. With the correct geometric configuration this phenomena will lead to droplet break up. Second, liquid-gas interfaces can enhance heat and mass transfer. If air bubbles, larger than the channel diameter, are confined to a liquid filled microfluidic channel they will elongate into plugs. These plugs are surrounded by a thin liquid film, which causes the bubbles to faster than the liquid creating a recirculating wake behind the bubble. This can be used to enhance mixing in the channel but it can also increase the heat and mass transfer between the liquid and channel wall. Third, solid-liquid interfaces can enhance and control boiling. In nucleate pool boiling, single bubbles form and depart from the wall. The frequency and size of the bubbles at departure can be influenced by the surface wettability. By patterning surfaces with wetting and non-wetting regions the growth of bubbles can be controlled to enhance the heat transfer.